This invention relates to a nematic liquid crystal composition used for display devices, particularly those of a time-division (or time sharing) driving system.
It is said that the most desirable liquid crystal materials (including compounds and compositions) used for field-effect type liquid crystal display elements, such as for example twisted nematic type (TN type) liquid crystal display elements, are those which meet the following three requirements:
First requirement: good adaptability to the orientation controlling section. PA1 Second requirement: operability over a wide temperature range. PA1 Third requirement: good responsiveness over a wide temperature range, particularly at low temperatures.
Various types of liquid crystal materials for display elements, particularly those of a time-division driving system, such as Schiff base type, ester type, biphenyl type, azoxy type, etc., have been proposed to date. The azoxy type liquid crystal materials have excellent temperature characteristic (small in .DELTA.T), that is, they are very limited in variation of threshold voltage with change of temperature and, as explained later, they provide an operational margin M of greater than 10% under the 1/3 bias and 1/3 duty time-division driving conditions. The azoxy liquid crystal materials are represented by the following general formula: ##STR3## These materials possess per se weakly negative dielectric anisotropy and are usually used as a mixed system with a nematic liquid crystal compound having positive dielectric anisotropy (N.sub.p). But these axoxy type liquid crystal materials are colored (in yellow) when they absorb a part of visible light. Also, they show the maximum light absorption at 350 nm and undergo the following photochemical reaction owing to the wavelength around such level: ##STR4## A non-liquid crystal compound is produced by such photochemical reaction and this new product changes the color of the liquid crystal from yellow into red. Usually, electric resistance of the liquid crystal is also sharply lowered. Therefore, in actual use of such azoxy type nematic liquid crystals, these is a need to adapt a 500 nm cut filter in the device (element) so as to avoid photo-deterioration that might be caused by the sunlight or fluorescent light. This use of a filter naturally complicates the mechanism of the device (element).
Other types of liquid crystals which are resistant to such photo-deterioration, such as Schiff base type, biphenyl type, ester type, etc., have been noticed for their availability as white display material and their adaptation to such display devices has been debated.
The biphenyl type liquid crystals are credited with high chemical stability as they are highly resistant to light, water and oxygen. However, most of the known biphenyl type materials which form liquid crystal at room temperature are the ones having positive dielectric anisotropy, and few are known of the negative equivalents which are liquid crystal at room temperature and practically useful. Therefore, there are only a few kinds of liquid crystal compounds which can form a mixed system with biphenyl type alone. Also, because of a not so high value of positive dielectric anisotropy, a wide range adjustment of the threshold value is hardly possible with these materials, and further, such threshold voltage has high temperature dependency, so that these materials are generally considered unsuited for time-division driving.
The ester type liquid crystal compounds have relatively good chemical stability and there are known many kinds of single liquid crystal compounds of positive or negative dielectric anisotropy. However, threshold voltage of these compounds has relatively high temperature dependancy and their viscosity is also considerably high, so that generally these compounds can hardly meet the afore-said second and third requirements.
The Schiff base type liquid crystal compounds have better properties than the ester type, but because of strong hydrolytic disposition, matching with the packing portion of the display element is often required for their use.
Individual liquid crystal materials are disclosed in, for example, U.S. Pat. Nos. 4,137,192 and 4,147,651, Molecular Crystals and Liquid Crystals 22, 285-299 (1973), J. Org. Chem. 38, 3160-3164 (1973), East German Pat. No. 105,701, etc., but their special combinations are not known yet.
The present inventors have already found that a liquid crystal system which meets the afore-said second and their requirements can be obtained by using a nematic liquid crystal compound with negative dielectric anisotropy (Nn type liquid crystal compound) as matrix and adding a suitable amount of a nematic liquid crystal compound with positive dielectric anisotropy (N.sub.p type liquid crystal compound) and/or its homologue (the term "homologue" is used to refer to the materials which are analogous in molecular structure to the positive nematic liquid crystal compounds, and such materials are hereinafter referred to as N.sub.p type liquid crystal homologue), and that such liquid crystal system can be obtained from Schiff base type liquid crystal compounds or cyclohexanecarboxylic acid-trans-4'-alkoxyphenyl esters, but such system is still unsatisfactory.